Passive array gain in shallow water

Detecting sound in the ocean is greatly improved by hydrophone array processing. Until recently, operating large arrays required cumbersome data acquisition systems and processing computers. Thanks to miniaturisation of electronics and improvements in computing power, array size is now limited by practicalities of deploying large physical apertures rather than the difficulties of the associated electronics. This allows improving signal to noise ratio, in particular against ambient noise. An important component of ambient noise is noise originating from the sea surface, which is strongly anisotropic. The detection performance of an array can be quantified by the directivity index which is easy to compute but difficult to measure or the array gain which is easily measurable but challenging to model. Directivity index is defined for isotropic noise, while array gain, which determines the actual performance of the system in a given environment, is defined for any noise directionality Extending the vertical or horizontal aperture of an array may result in any an increased directivity index but array gain will also be affected by signal coherence and noise anisotropy. These two characteristics are in turn influenced by all the environmental parameters related to acoustic propagation (water column, surface and seafloor parameters). In this paper, we use semi analytical formulas of array gain to study array gain against surface noise in simple shallow water environments. In particular we quantify the relation between array gain, aperture and geo-acoustic parameters and compare it with directivity index.